1. Field of the Invention
This invention relates to a method for removing nitrogen-oxides from gas, particularly to a method for removing nitrogen-oxides from gas by microwave catalytic reaction by microwave catalysis or by microwave irradiation.
2. Description of the Related Art
Nitrogen-oxides NOx are the main pollutants which do a great harm to human health, and NO is 95% by weight of the nitrogen-oxides.
Typically, nitrogen-oxides are removed by selective catalytic reduction process in which ammonia is used as a reducer (NH3-SCR process). The NH3-SCR process is the most widely used method for removing nitrogen-oxides, and reaches a denitrification rate of 80-90% at a low temperature. However, for the NH3-SCR process, the consumption of reducer is high, the catalyst is easy to be poisoned, the requirements for equipment are high, and the denitrification rate is low.
The NO decomposition process, which is cost-effective and does not require reducers such as NH3, CO, and CH4 and result in pollution, is considered as the most promising method. The decomposition reaction of NO is as follows: 2NON2+O2. In theory, it is possible that the conversion rate of NO into N2 and O2 reaches 99%, but the activation energy is as high as 364 kl/mol. Therefore, the key point of the NO decomposition process is to find out a suitable catalyst and an efficient energy to reduce the activation energy of the decomposition reaction of NO.
So far, Cu—ZSM-5 molecular sieve catalyst is the best catalyst for decomposing NO at a low temperature. The optimum activation temperature of the Cu—ZSM-5 molecular sieve catalyst is 450-550° C. However, the highest conversion rate of NO is about 70% because the activity of Cu—ZSM-5 molecular sieve catalyst is inhibited by oxygen. Modified Cu—ZSM-5 molecular sieve catalysts, such as Ce—Cu—ZSM-5, are less inhibited by oxygen, and the conversion rate of NO thereof is 75%. In conclusion, the NO decomposition process does not achieve a high denitrification rate, and is greatly influenced by oxygen.